I am hoping the heartbeat and the WiFi performance bits are not related. If I were testing this, and attempting to get to the bottom of the issue, I would plug a device into the CP12, and likely use PingPlotter and try to monitor three distinct points in the network.
- ping from the device to the CP12's IP address
- ping from the device to a non-sleeping client on the wifi (this can be tricky)
- ping from the device to each of the APs
- ping the "gateway"
- ping the wan Gateway
With Ping Plotter, you'll soon get graphs relating to the interrupts. With this data, you should be able to cross-reference missed ping events against the "heart Beat Lost" messages, and the graphs will give you a visual way to see which devices are affected.
Also, has your Wi-Fi dropped since you made the settings change?